Welcome to Rappler, a social news network where stories inspire community engagement and digitally fuelled actions for social change. Rappler comes from the root words "rap" (to discuss) + "ripple" (to make waves).

SUPER TUESDAY. People wait in line to vote at a polling center in Edmond, Oklahoma, USA, March 1, 2016. Photo by J Pat Carter/EPA

ARLINGTON, Virginia, USA (UPDATED) – Americans cast ballots Tuesday, March 1, on the most pivotal day of the White House primary season, with frontrunners Hillary Clinton and Donald Trump hoping to wipe out all rivals for their party nominations.

Millions of people are taking part in Super Tuesday – a series of primaries and caucuses in a dozen states stretching from Massachusetts and Virginia on the east coast to Texas and all the way to Alaska. (FAST FACTS: US 'Super Tuesday')

If Democrat Clinton and Republican Trump – an outspoken billionaire who has tapped into a vein of conservative rage at conventional politics – win big, they could be well on their way to the nominations, spelling doom for their challengers.

Polling stations opened first in Virginia where a steady stream of voters stopped to cast ballots on a day like few others on the calendar leading up to the November presidential election.

In the swing state's suburban, middle-of-the-road north, some Democrats considered crossing party lines to cast protest ballots against Trump.

"Normally I would vote for the Democrats, but I am more afraid of Trump. I still don't know how I will vote," said one woman as she walked into a polling station in Arlington.

Steve Slye, who runs an audio visual company in Arlington, said he voted for Ohio Governor John Kasich, who has run a more optimistic campaign than Trump but has failed to generate the enthusiasm necessary to topple the frontrunner.

"He's the adult in the room, to me," he said.

Trump's main Republican rivals, Senators Marco Rubio and Ted Cruz, have been frantically trying to halt the real estate magnate's march toward the nomination, seeking to unite the party against the man they see as a non-conservative political interloper.

But it might be too little too late, with polls showing Trump in a commanding lead in most Super Tuesday states and beyond.

Former secretary of state Clinton, coming off a blowout weekend win in South Carolina, is also leading her rival Senator Bernie Sanders by similarly significant margins.

Even as Clinton made her final case to voters Tuesday in Minnesota, she appeared to tilt toward the general election matchup, assailing Republicans "running their campaigns based on insults."

Asked if Trump would be the eventual nominee, she told reporters "he could be on the path." But "whoever they nominate, I'll be prepared to run against if I'm fortunate to be the nominee."

A new CNN/ORC poll found that both Clinton and Sanders would easily defeat Trump if the general elections were held now.

If Rubio or Cruz were the Republican nominee, Clinton would face a much closer race. Strikingly, Sanders, a self-described democratic socialist, topped all 3 Republican candidates by wide margins, the poll showed.

The Florida senator warned supporters in Tennessee Monday, February 29, that Democratic groups will jump on Trump "like the hounds of hell" if he wins the nomination.

Trump received a stern rebuke Tuesday from House Speaker Paul Ryan over his failure to immediately denounce the support of David Duke, a former leader of the Ku Klux Klan.

"If a person wants to be the nominee of the Republican Party, there can be no evasion and no games. They must reject any group or cause that is built on bigotry," Ryan, the top Republican in Congress, told reporters.

"This party does not prey on people's prejudices."

Trump remains in the driver's seat. He is leading in polls in at least 8 of the 11 Super Tuesday states, and expanding his lead nationally with CNN/ORC giving him 49% support.

Rubio is a distant second at 16%, with Cruz one point further behind.

Trump has punched back against "Little Marco," mocking him for sweating on the campaign trail and warning that he could not stand up to strong men like Russian President Vladimir Putin.

On Tuesday, Trump derided the "horrible" job his rivals have done in their home states.

Furious electorate

Trump's inflammatory rhetoric, in which he has accused Mexico of sending rapists across the border, mocked women and the disabled, urged a ban on Muslims entering the country, and eagerly advocated the use of torture, would have been the undoing of a normal candidate.

But the 2016 cycle has been anything but normal, with a furious electorate keen to back an outsider who scorns the political establishment.

"I'm representing a lot of anger out there," Trump told CNN.

"We're not angry people, but we're angry at the way this country's being run."

If Trump sweeps the South, where many of the Super Tuesday races are taking place, it could be lights out for his Republican challengers.

Texas is the largest prize on Tuesday, and Cruz is banking on winning his home state. He trails in nearly all other Super Tuesday states.

Almost 600 Republican delegates are up for grabs Tuesday, nearly half the 1,237 needed to secure the nomination.

Some 865 Democratic delegates are at stake, 36% of those needed to win. – Magan Crane, with Michael Mathes in Washington DC, AFP / Rappler.com

Would you like to share your vote?

Welcome to Rappler, a social news network where stories inspire community engagement and digitally fuelled actions for social change. Rappler comes from the root words "rap" (to discuss) + "ripple" (to make waves).